Regenerative fuel cell systems provide a reversible electrochemical process for using electricity to convert water into hydrogen and oxygen and then reversing the process to combine the hydrogen and oxygen to create electricity. Typically, these systems have a membrane electrode assembly (“MEA”) that includes an ion conducting polymer membrane sandwiched between two electrodes containing a catalyst material. In fuel generation, or electrolysis mode, the electrical current is passed through the electrodes, causing the water in contact with the anode catalyst to decompose into its base elements of hydrogen and oxygen. Due to the unique characteristics of the polymer membrane, hydrogen ions are driven by the electrical current to the opposite cathode electrode where in the presence of the second catalyst, the hydrogen ions recombine with electrons to form hydrogen gas. In electrical generation, or fuel cell mode, the process is reversed. Hydrogen gas is introduced to the cathode electrode that decomposes the hydrogen into a hydrogen ion and an electron. The hydrogen ion passes through the polymer membrane and combines with oxygen to form water at the anode electrode. Regenerative fuel cell systems are commonly divided into two categories: unitized regenerative fuel cells and discrete regenerative fuel cells.
In a unitized regenerative fuel cell system (“URFC”), a single electrochemical cell used to generate both the hydrogen gas and the electricity using the same MEA. This type of system has several advantages in that a single component can be used for both generating modes, leading a smaller lighter system. However, since the process must be completely reversible within the component, neither of the electrodes can be optimized for a particular generation mode resulting in efficiency losses. Due to efficiency issues and cost considerations, URFC″s are typically limited to aerospace or geographically remote applications where the size and weight parameters are paramount.
In contrast to the URFC, a discrete regenerative fuel cell (“DRFC”) system is designed to utilize two electrochemical cell components: an electrolysis cell for generating hydrogen; and a fuel cell for generating electricity. By dividing the generation modes between two components each of the individual cell components can be optimized for its particular purpose. This leads to a more efficient and cost effective solution than can be currently accomplished with a URFC. Applications for the DRFC include backup or emergency power systems for buildings or telecommunications facilities such as cellular phone towers.
What is needed in the art is a cost effective and efficient regenerative fuel cell system that is adaptable to the changing needs of an application and which provides a greater degree of independent operation than had been previously available.